Thirteen people are dead and at least eight others were injured after a gunman opened fire at the Washington Navy Yard on Monday, officials said, spreading fear and chaos across the region as authorities sought to contain the panic.

The incident, in which the death toll rose almost hourly, represents the single worst loss of life in the District since an airliner plunged into the Potomac River in 1982, killing 78.

D.C. Police Chief Cathy L. Lanier and Mayor Vincent C. Gray announced the mounting number of casualties in a series of news conferences. The suspected shooter, identified by the FBI as Aaron Alexis, 34, living in Fort Worth, is among the dead. Alexis was a military contractor, one official said.

Hours after the rampage began it remained unclear whether the shooting was the act of a lone gunman, or if other shooters were involved. Lanier initially said authorities were looking for two more potential shooters dressed in military style clothing. Shortly after she announced a detailed description of two suspects, city officials said one had been located and cleared. And at a 10 p.m. news conference she said police were comfortable the shooting had been committed by one person.

Police are asking anyone with information on the suspect to call 202-727-9099.

Lanier lifted the shelter in place order for the neighborhood around the Navy Yard that had been in place for much of the day.

Gray said those killed by the shooter ranged in age from 46-73 years old. He said the families of seven of the victims had been notified. Those victims were identified as: Michael Arnold, 59; Sylvia Frasier, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Bernard Proctor, 46; and Vishnu Pandit, 61.

The families of the six other victims were in the process of being notified.

Lanier said one of those killed was a District resident, but she did not identify the person.

Gray said another eight people were injured in the attack. He said three people were shot, including Metropolitan Police Officer Scott Williams. Gray said the officer was doing well.

The other five had either stress related injuries, and at least one was someone who fell.

Vice Adm. Bill French earlier had said that the number of injured was 14.

[For up to the minute information about the shooting, check The Post’s live blog.]

Gray said no motive is known yet. He said there is no reason to believe it was an act of terrorism, though he added that he could not rule it out.

As of 8:30 p.m. Monday, Navy officials said about 2,000 people remained on base, and that it could take another 3-4 hours before everyone was cleared to leave.

The FBI was still interviewing every person leaving the base out of concern that a second suspect may still be at large, French said. And SWAT teams are still finding people hiding in places on the base, where some had remained hunkered down since the initial attack early Monday morning. One city officials said that shortly before 7 p.m., officers found an employee hiding in a locker, where the employee had been for nearly 11 hours.

Throughout the day, people had been warned to stay in their homes and offices on the Naval Base as the incident unfolded.

Alexis was armed with an assault rifle and a handgun, two law enforcement officials said. One said he also had a shotgun. One official said all the weapons have not been accounted for.

The first, sketchy details about the suspect offered few hints about what may have gone wrong.

Alexis grew up in Brooklyn with his mother, Cathleen, and father, Anthony Alexis, according to his aunt Helen Weeks.

“We haven’t seen him for years,” Weeks said of her nephew in a telephone interview. “I know he was in the military. He served abroad. I think he was doing some kind of computer work.”

Alexis spent nearly four years in the Navy as a full-time reservist from May 2007 until he was discharged in January 2011, according to a summary of his personnel records released by Navy officials at the Pentagon.

The officials said they were still researching whether Alexis had been employed as a defense contractor or a civilian employee of the Navy, and were uncertain if he was assigned to work at the Navy Yard.

He achieved his final rank of Aviation Electrician’s Mate 3rd Class in December 2009. Officials said they did not immediately know the reasons for his discharge.

The carnage began around 8 a.m. when the U.S. Navy said that three shots were fired at Building 197, headquarters of the Naval Sea Systems Command. About 3,000 people work in the building. As the noise that some thought sounded like construction work continued, the realization set in that a gunman was firing on them.

Rick Mason, a program management analyst who is a civilian with the U.S. Navy, told the Associated Press that a gunman was shooting from a fourth floor overlook in the hallway outside his office. He said the gunman was aiming at people in the building’s first floor cafeteria.

Terry Durham said that as she and co-workers were evacuating, she saw a man down the hall raise a rifle and fire toward them, hitting a wall.

“He didn’t say a word,” said her co-worker, Todd Brundage.

One man who said he was at his desk on the second floor when the shooting began recalled hearing a loud noise “like someone dropping an old metal desk.” The man, who declined to give his name, said there was a pause, then several noises close together and he realized the danger: “There’s a shooter in the building. I started walking toward the door and I heard people running down the hall.”

Employees described the chaos, as a fire alarm sounded and people shouted, “Where is he? Where is he?”

Gregory Dade, a Navy contractor, said he and a co-worker locked themselves in a second floor office of Building 197 as soon as the shooting went on, in fits and stops. Dade, called it “terrifying.”

He heard a woman scream, glass crashing and a series of gun shots. Then he heard shouting: “Get down! Get down! This is the police.”

Finally, about 11 a.m., he and a co-worker made a break for it.

At an exit, they noticed a trail of blood running to the next building.

Tim Jirus, a Navy commander who works in Building 197, said that as he was evacuating the building he saw a co-worker who had been shot getting into a police car, and heard more shots fired from inside his workplace.

A far worse sight lay just ahead. Jirus went to an alley where he thought he would be safe, and talked briefly with a man there about what was going on. Jirus said he heard two gunshots, loudly echoing off the building. It caused him to spin around to look for the source of the noise. But when he turned back, he looked down and saw the man he had been conversing with lying on the ground, shot in the head.

Uncertain where the shooter was, he ran.

“I was just lucky,” he said. “The other person was shorter than me. There were two shots, he got that guy, he didn’t get me. . . . The randomness of it — standing right next to me, one person gets shot.”

At least two police officers were among those shot. Lanier said the D.C. officer worked in the K-9 unit and has been on the force 23 years. She said she has known and worked closely with him for "many, many years."

"He's got a stellar record," she said.

"He's got a pretty serious injury, but right now his family is here with him and he's in good spirits," Lanier added. "He's just very grateful for all the other responders who helped get him out of that building and get the medical attention he needed."

The other was said to be a base officer. Earlier in the say officials at MedStar Washington Hospital Center said his chances for survival were good.

Janis Orlowski, the chief medical officer at Washington Hospital Center, said three victims in all were brought to the center, all in critical condition but alert, responsive and able to talk with doctors. The victims were also able to speak briefly to law enforcement officers before undergoing surgery or treatment, she said.

The other two victims at the hospital were female civilians, Orlowski said at a news conference. All are likely to survive.

The impact of the incident rippled across town, forces schools, offices and homes into an enforced lockdown.

Senate buildings on Capitol Hill went on lockdown at about 3 p.m., with no one allowed to enter or leave the building, though it was partially lifted toward the end of the day to allow staffers to go home.

The Nationals, whose ballpark is near the Navy Yard, postponed a Monday night game. Instead, they will play a double header Tuesday at 1 p.m. and 7 p.m.

Ten public and charter schools and a public school administration building in the District went on lockdown, and flights out of Reagan National Airport were briefly halted, causing delays even after they began departing again.

Adm. Jonathan W. Greenert, the chief of Naval Operations, was evacuated from his residence at the Navy Yard complex shortly after the first report of shots fired, Navy officials said.

Greenert, a four-star admiral and member of the Joint Chiefs of Staff, was safely evacuated to the Pentagon along with his wife, Darleen, said Cmdr. Ryan Perry, a Navy spokesman.

Police closed the 11th Street Bridge as well as M Street SE between Second and Fourth streets SE because of the shooting. Entrances to the Navy Yard Metro station remain open.

As helicopters circled overhead and emergency vehicles continued to rush to the scene, crowds of onlookers gathered on sidewalks and at a construction site near the Navy Yard, but police pushed them back, yelling at them to keep a distance from the grounds.

President Obama on Monday expressed sympathy for the victims of the shooting and said justice must be sought.

“I’ve made it clear to my team that we want the investigation to be seamless, so that local and federal authorities are working together,” he said.

As a long day drew to a close, weary and stunned Navy personnel filtered into Capitol Hill in their tan uniforms, blue camouflage fatigues and civilian clothes, many of them saying little, just wanting to get home after a day spent running or hiding, and then waiting.

Tom Dick-Peddie, 46, was at work in another building in the Navy Yard and said they were on lockdown for about an hour and a half, then they were evacuated to another building. Shortly after 3 p.m. they were allowed to leave.

Kate O’Neill of Arlington and Stephanie Cates-Harman of Columbia, who work in the office of the general counsel, said their office had been ordered to shelter in place until 2 p.m. While some people were very upset, and others were calm, and tried to contact relatives to assure them that they were okay.

“Texts were going fast and furious,” O’Neill said.

Under Secretary for the Navy, Juan Garcia, said Navy Yard would reopen tomorrow for essential personnel only. Most employees would be encouraged to telecommute. Garcia said it was unclear who the base would reopen in its entirety.

Garcia said the Navy has set up a counseling line for anyone struggling to cope with Monday's attack. That number is 1-800-222-0364.

Both Gray and Lanier said the streets around the Navy Yard would be opened by about 5:30 a.m. Tuesday.

Carol Morello is the diplomatic correspondent for The Washington Post, covering the State Department.

Clarence Williams is the night police reporter for The Washington Post and has spent the better part of 13 years standing next to crime scene tape, riding in police cars or waking officials in the middle of night to gather information about breaking news in and around Washington.